Florida Keys Acropora

i took this shot last week and i was wondering if anything could be done to make it better. i really like close of of corals and i would like to be better at it. also would anyone have an idea of what that blue line is across the top of the picture? its in a bunch of my shots, but not all of them, so i'm not sure what could have caused it.

shot was taken with a D100 w/ 60mm f/2.8, Nexus housing, one DS125 strobe

That blue line might be that you have exceeded your flash synchronisation speed. This is 1/180th on the D100. You may have taken this picture at 1/250th. Do you have the EXIF?

The blue line is being caused by the second/rear curtain of the shutter already coming across the image as the shutter is fired.

Alex

p.s. The shot is simple and effective. The foreground stands out well from the background. The only critique I have is that I would like to see both tips of the staghorn inside the frame. The right one ios slightly clipped.

It might be worth thinking about using two strobes for a shot like this. The shadows created by using a single strobe can be dramatic, but here I think the viewer just wonders why one arm of the coral is well-illuminated and the other not. One strobe can be set to a lower power to avoid lighting that is too even. If you're limited to only one strobe, you might consider positioning the strobe above the port instead off to the side.

I think I'd also like this to be a bit sharper, or perhaps a bit more depth of field, but still keeping the background blurred, which helps the main subject stand out.

the shooting data was f/18, 1/250 so i guess that would be the shutter curtain making its way back into the frame.

i have been thinking about a second strobe, but i really can't afford it at the moment. i did have the strobe placed above the port, but i guess i wasn't looking at the subject straight on, hence the uneven lighting.

Yep, I've had that happen to me as well. The D100 doesn't "know" it has a flash attached, so it will fire at any shutterspeed you set - which is not good. With the Fuji S2, when you exceed the flash speed, it will default to 1/125th.